Charge storage device, method of making same, method of making an electrically conductive structure for same, mobile electronic device using same, and microelectronic device containing same

ABSTRACT

In one embodiment a charge storage device includes first ( 110 ) and second ( 120 ) electrically conductive structures separated from each other by a separator ( 130 ). At least one of the first and second electrically conductive structures includes a porous structure containing multiple channels ( 111, 121 ). Each one of the channels has an opening ( 112, 122 ) to a surface ( 115, 125 ) of the porous structure. In another embodiment the charge storage device includes multiple nanostructures ( 610 ) and an electrolyte ( 650 ) in physical contact with at least some of the nanostructures. A material ( 615 ) having a dielectric constant of at least 3.9 may be located between the electrolyte and the nanostructures.

FIELD OF THE INVENTION

The disclosed embodiments of the invention relate generally to chargestorage devices, and relate more particularly to capacitors, includingelectric double-layer capacitors.

BACKGROUND OF THE INVENTION

Charge storage devices, including batteries and capacitors, are usedextensively in electronic devices. In particular, capacitors are widelyused for applications ranging from electrical circuitry and powerdelivery to voltage regulation and battery replacement. As capacitortechnology has continued to develop, several types have emerged. Forexample, electric double-layer capacitors (EDLCs), also referred to asultracapacitors (among other names), are characterized by high energystorage and power density, small size, and low weight and have thusbecome promising candidates for use in several applications.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed embodiments will be better understood from a reading ofthe following detailed description, taken in conjunction with theaccompanying figures in the drawings in which:

FIGS. 1 and 2 are cross-sectional views of a charge storage deviceaccording to embodiments of the invention;

FIG. 3 is a cross-sectional scanning electron microscope image of apiece of porous silicon according to an embodiment of the invention;

FIG. 4 is a cross-sectional representation of an electric double layerwithin a channel of a charge storage device according to an embodimentof the invention;

FIG. 5 is a cross-sectional view of a channel within a charge storagedevice showing various layers and structures according to embodiments ofthe invention;

FIG. 6 is a cross-sectional view of a charge storage device according toanother embodiment of the invention;

FIG. 7 is a flowchart illustrating a method of making an electricallyconductive structure for a charge storage device according to anembodiment of the invention;

FIG. 8 is a perspective view of a relatively thick electricallyconductive structure according to an embodiment of the invention;

FIG. 9 is a flowchart illustrating a method of making a charge storagedevice according to an embodiment of the invention;

FIG. 10 is a block diagram representing a mobile electronic deviceaccording to an embodiment of the invention; and

FIG. 11 is a block diagram representing a microelectronic deviceaccording to an embodiment of the invention.

For simplicity and clarity of illustration, the drawing figuresillustrate the general manner of construction, and descriptions anddetails of well-known features and techniques may be omitted to avoidunnecessarily obscuring the discussion of the described embodiments ofthe invention. Additionally, elements in the drawing figures are notnecessarily drawn to scale. For example, the dimensions of some of theelements in the figures may be exaggerated relative to other elements tohelp improve understanding of embodiments of the present invention. Thesame reference numerals in different figures denote the same elements,while similar reference numerals may, but do not necessarily, denotesimilar elements.

The terms “first,” “second,” “third,” “fourth,” and the like in thedescription and in the claims, if any, are used for distinguishingbetween similar elements and not necessarily for describing a particularsequential or chronological order. It is to be understood that the termsso used are interchangeable under appropriate circumstances such thatthe embodiments of the invention described herein are, for example,capable of operation in sequences other than those illustrated orotherwise described herein. Similarly, if a method is described hereinas comprising a series of steps, the order of such steps as presentedherein is not necessarily the only order in which such steps may beperformed, and certain of the stated steps may possibly be omittedand/or certain other steps not described herein may possibly be added tothe method. Furthermore, the terms “comprise,” “include,” “have,” andany variations thereof, are intended to cover a non-exclusive inclusion,such that a process, method, article, or apparatus that comprises a listof elements is not necessarily limited to those elements, but mayinclude other elements not expressly listed or inherent to such process,method, article, or apparatus.

The terms “left,” “right,” “front,” “back,” “top,” “bottom,” “over,”“under,” and the like in the description and in the claims, if any, areused for descriptive purposes and not necessarily for describingpermanent relative positions. It is to be understood that the terms soused are interchangeable under appropriate circumstances such that theembodiments of the invention described herein are, for example, capableof operation in other orientations than those illustrated or otherwisedescribed herein. The term “coupled,” as used herein, is defined asdirectly or indirectly connected in an electrical or non-electricalmanner. Objects described herein as being “adjacent to” each other maybe in physical contact with each other, in close proximity to eachother, or in the same general region or area as each other, asappropriate for the context in which the phrase is used. Occurrences ofthe phrase “in one embodiment” herein do not necessarily all refer tothe same embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

In one embodiment of the invention, a charge storage device comprises afirst electrically conductive structure and a second electricallyconductive structure separated from each other by an electricalinsulator, wherein at least one of the first electrically conductivestructure and the second electrically conductive structure comprises aporous structure containing multiple channels, and wherein each one ofthe channels has an opening to a surface of the porous structure.

Ultracapacitors and similar high-surface-area charge storage devices canbe used in microelectronics to store energy, for electrical bypassing inelectric circuits, as part of circuitry for power delivery, as a memorystorage element, and for a host of other functions. An advantage ofultracapacitors over batteries is that ultracapacitors can be chargedand discharged quickly because they do not rely on chemical reactions tostore energy, and they don't degrade significantly over theirlifetime—even when charged and discharged rapidly. Ultracapacitors arealso less sensitive to temperature than are batteries.

The development path of ultracapacitors is such that they appear likelyto eventually achieve greater energy density (both in terms of energyper kilogram (kg) and of energy per liter) than batteries.Ultracapacitors can thus be used in conjunction with batteries in orderto protect the batteries from high power bursts (thereby extending thebattery lifetime). Furthermore, the electrodes in batteries can be madethinner if the ultracapacitors can provide the high power demands.Alternatively, ultracapacitors may make sense as a battery replacement.Embodiments of the invention are capable of increasing the energydensity of ultracapacitors by several orders of magnitude, for exampleby increasing the electrode surface area with nanomaterials coated withhigh-k dielectric materials, as will be discussed in detail below.

Referring now to the drawings, FIGS. 1 and 2 are cross-sectional viewsof a charge storage device 100 according to embodiments of theinvention. As illustrated in FIGS. 1 and 2, charge storage device 100comprises an electrically conductive structure 110 and an electricallyconductive structure 120 separated from each other by an electricalinsulator. This electrical insulator could take one of various forms, asdiscussed in more detail below. At least one of electrically conductivestructures 110 and 120 comprises a porous structure containing multiplechannels, each one of which has an opening to a surface of the porousstructure. In the illustrated embodiments both electrically conductivestructure 110 and electrically conductive structure 120 comprise such aporous structure. Accordingly, electrically conductive structure 110comprises channels 111 with openings 112 to a surface 115 of thecorresponding porous structure and electrically conductive structure 120comprises channels 121 with openings 122 to a surface 125 of thecorresponding porous structure. In an embodiment where only one ofelectrically conductive structures 110 and 120 comprises a porousstructure with multiple channels, the other electrically conductivestructure can be, for example, a metal electrode or a polysiliconstructure.

Various configurations of charge storage device 100 are possible. In theembodiment of FIG. 1, for example, charge storage device 100 comprisestwo distinct porous structures (electrically conductive structure 110and electrically conductive structure 120) that have been bondedtogether face-to-face with an intervening separator 130. As anotherexample, in the embodiment of FIG. 2 charge storage device 100 comprisesa single planar porous structure in which a first section (electricallyconductive structure 110) is separated from a second section(electrically conductive structure 120) by a trench 231 containingseparator 130. One of the electrically conductive structures will be thepositive side and the other electrically conductive structure will bethe negative side. Separator 130 permits the transfer of ions but doesnot allow the transfer of fluid such as would be found in anelectrolyte.

FIG. 2 shows a small bridge of material connecting electricallyconductive structure 110 and electrically conductive structure 120. Ifleft unaddressed, this bridge may act as an electrical short between thetwo electrically conductive structures. There are several possiblesolutions, however. For example, the bridge may be removed using apolishing operation. Alternatively, the electrically conductivestructures may be formed in a heavily-doped top layer or region of awafer while the trench extends down to an underlying lightly-dopedsubstrate that is not a very good conductor. Or a silicon-on-insulatorstructure may be used.

As an example, the porous structure of electrically conductivestructures 110 and 120 can be created by a wet etch process in which aliquid etchant applied to a surface of the electrically conductivestructures etches away portions of the electrically conductive structurein a way that it at least somewhat similar to the way water is able tocarve channels in rock. This is why each one of the channels has anopening to the surface of the electrically conductive structure; the wetetch method is incapable of creating fully-enclosed cavities, i.e.,cavities with no opening to the surface, like an air bubble trappedinside a rock, within the porous structure. This is not to say thatthose openings cannot be covered with other materials or otherwiseclosed up because of the presence of or addition of other materials—thatis in fact likely to occur in several embodiments—but, whether coveredor not, the described openings to the surface are a feature of eachchannel in each porous structure according to at least one embodiment ofthe invention. (One embodiment in which the openings may be covered upis one in which a layer of epitaxial silicon as a location for circuitryor other wiring is grown on top of the channels). Porous structuresaccording to embodiments of the invention can be fabricated with veryprecise and uniform pore size control (in contrast to active carbon).This allows fast charging (pore size may be optimized in order to becompatible with the size of the ions) and also improves the capacitance(no area will be malfunctioning). This would also allow narrowdistribution of voltage fluctuation.

It should be noted in connection with this discussion that porouscarbon, being formed in a manner different from that described above,has a different structure—one that is characterized by fully-enclosedcavities having no surface openings. As a result, porous carbon is notsuitable—or at least not as desirable—for at least certain embodimentsof the invention (although it should be mentioned here that certainother embodiments (such as, for example, the thick electricallyconductive structure described below) may contain fully-enclosedcavities). It should also be noted that the FIG. 1 and FIG. 2 depictionsof the porous structures are highly idealized in that, to mention justone example, all of channels 111 and 121 are shown as only extendingvertically. In reality the channels would branch off in multipledirections to create a tangled, disorderly pattern that may looksomething like the porous structure shown in FIG. 3.

FIG. 3 is a cross-sectional scanning electron microscope (SEM) imageshowing a piece of porous silicon 300 according to an embodiment of theinvention. As illustrated, porous silicon 300 contains multiple channels311, some of which appear elongated vertically and some of which appearas roughly circular holes. The latter group represent channels for whichthe visible portion is oriented horizontally. It should be understoodthat channels 311 are likely to twist and turn along their lengths suchthat a single channel may have both vertical and horizontal portions aswell as portions that are neither completely vertical nor completelyhorizontal but fall somewhere in between.

With the right etchant, it should be possible to make porous structureshaving the described characteristics from almost any conductivematerial. As an example, a porous silicon structure may be created byetching a silicon substrate with a mixture of hydrofluoric acid andethanol. More generally, porous silicon and other porous structures maybe formed by such processes as anodization and stain etching.

Besides porous silicon, which has already been mentioned, some othermaterials that may be especially well-suited for charge storage devicesaccording to embodiments of the invention are porous germanium andporous tin. Possible advantages of using porous silicon include itscompatibility with existing silicon technology. Porous germanium enjoysa similar advantage as a result of existing technology for that materialand, as compared to silicon, enjoys the further possible advantage thatits native oxide (germanium oxide) is water-soluble and so is easilyremoved. (The native oxide that forms on the surface of silicon may trapcharge—which is an undesirable result—especially where the siliconporosity is greater than about 20 percent.) Porous germanium is alsohighly compatible with silicon technology. Possible advantages of usingporous tin, which is a zero-band-gap material, include its enhancedconductivity with respect to certain other conductive and semiconductivematerials. Other materials may also be used for the porous structure,including silicon carbide, alloys such as an alloy of silicon andgermanium, and metals such as copper, aluminum, nickel, calcium,tungsten, molybdenum, and manganese. A silicon-germanium alloy, forexample, will advantageously exhibit a much smaller volume differencethan a pure germanium structure.

Embodiments of the invention may make use of very narrow channels. Incertain embodiments (to be described in detail below), an electrolyte isintroduced into the channels. Molecules in the electrolyte may be on theorder of 2 nanometers (nm). In at least one embodiment, therefore, asmallest dimension of each one of the channels is no less than 2 nm soas to permit the electrolyte to flow freely along the entire length ofthe channels.

In the same or another embodiment the smallest dimension of each one ofthe channels is no greater than 1 micrometer (μm). This upper size limitfor the smallest dimension of the channels may be chosen for particularembodiments in order to maximize the surface area of the porousstructures of those embodiments. Smaller (e.g., narrower) channels leadto increased overall surface area for each electrically conductivestructure because a larger number of such narrower channels can fit intoan electrically conductive structure of a given size. Becausecapacitance is proportional to surface area, channels constrained insize in the manner described would likely, and advantageously, result incapacitors with increased capacitance. (The channels' other dimensions,e.g., their lengths, may also be manipulated in order to increasesurface area (or to achieve some other result)—i.e., longer channels maybe preferred over shorter ones—but otherwise are likely to be lesscritical than the smallest dimension discussed above.) In otherembodiments the smallest dimension of the channels may be larger than 1μm—perhaps as large as 10 μm or more. Although they would decrease thesurface area, such larger channels may provide more interior space inwhich to grow or otherwise form additional structures, if desired. Atleast one such embodiment is discussed below.

Charge storage device 100 further comprises an electrically conductivecoating 140 on at least a portion of the porous structure and in atleast some of channels 111 and/or channels 121. Such an electricallyconductive coating may be necessary in order to maintain or enhance theconductivity of the porous structure—especially where the porosity ofthe porous structure exceeds about 20 percent. As an example,electrically conductive coating 140 may be a silicide. As anotherexample, electrically conductive coating 140 may be a coating of metalsuch as, for example, aluminum, copper, and tungsten, or otherelectrical conductors such as tungsten nitride, titanium nitride, andtantalum nitride. Each of the listed materials has the advantage ofbeing used in existing CMOS technology. Other metals such as nickel andcalcium may also be used as electrically conductive coating 140. Thesematerials may be applied using processes such as electroplating,chemical vapor deposition (CVD), and/or atomic layer deposition (ALD).It should be noted here that a CVD process of tungsten is self-limiting,meaning that the tungsten will form a couple of monolayers and then stopgrowing. The resulting thin electrically conductive coating is exactlywhat is needed for embodiments of charge storage device 100 because itnever gets so thick as to seal off the channels and prevent the CVD gasfrom penetrating deeper into the channels. If desired, the porousstructure can also be doped with a dopant designed to increase theelectrical conductivity of the structure (boron, arsenic, or phosphorus,for example, for porous silicon; arsenic or gallium, for example, forporous germanium).

In one embodiment the electrical insulator separating electricallyconductive structure 110 from electrically conductive structure 120comprises a dielectric material. For example, one could make a veryhigh-capacitance capacitor using a porous silicon electrode oxidizedwith silicon dioxide (SiO₂) along with a metal or polysilicon structureas the other electrode. The very high surface area of the porous siliconwould be a major contributor to the high capacitance that could beachieved with such a capacitor.

The capacitance could be increased still further—even significantlyincreased—by placing an electrolyte 150 in physical contact with theporous structure. Electrolyte 150 (as well as other electrolytesdescribed herein) is represented in the drawings using a randomarrangement of circles. This representation is intended to convey theidea that the electrolyte is a substance (liquid or solid) containingfree ions. The circles were chosen for convenience and are not intendedto imply any limitation as to the electrolyte components or qualities,including any limitation with respect to the size, shape, or number ofthe ions. A typical, though not the only, type of electrolyte that maybe used in accordance with embodiments of the invention is an ionicsolution.

In an embodiment where electrolyte 150 is used, the electrical insulatorseparating electrically conductive structure 110 from electricallyconductive structure 120 can be an electric double layer that is createdby the presence of the electrolyte. This electric double layer, depictedschematically in FIG. 4, can complement or replace the dielectricmaterial described above. As illustrated in FIG. 4, an electrical doublelayer (EDL) 330 has been formed within one of channels 111. EDL 330 ismade up of two layers of ions, one of which is the electrical charge ofthe sidewalls of channel 111 (depicted as being positive in FIG. 4 butwhich could also be negative) and the other of which is formed by freeions in the electrolyte. EDL 330 electrically insulates the surface,thus providing the charge separation necessary for the capacitor tofunction. The large capacitance and hence energy storage potential ofelectrolytic ultracapacitors arises due to the small (approximately 1nm) separation between electrolyte ions and the electrode.

It should be noted that when charge storage device 100 is dischargedthen the EDL dissipates. This means that under some circumstances—wherethe EDL replaces the dielectric layer, for example—electricallyconductive structures 110 and 120 may for a time not be separated fromeach other by an electrical insulator—at least not the one embodied inthe EDL. References herein to “a first electrically conductive structureand a second electrically conductive structure separated from each otherby an electrical insulator” specifically include situations where, asdescribed above, the electrical insulator is only present when thecharge storage device is electrically charged.

In some embodiments electrolyte 150 is an organic electrolyte. As oneexample, the electrolyte can be a liquid or solid solution of organicmaterials such as tetraethylammonium tetrafluoroborate in acetonitrile.Other examples include solutions based on boric acid, sodium borate, orweak organic acids. Alternatively, (non-organic) water could be used asthe electrolyte, but this may pose a safety risk in that water may boiland form a gas if the capacitor exceeds a certain temperature, possiblycausing the capacitor to explode.

As mentioned above, high energy density is a desired characteristic forcapacitors. However, a typical electrical double layer can withstandonly a relatively low voltage—perhaps 2 or 3 volts—and this limits theenergy density that can be achieved in practice. In order to increasethe achievable energy density, embodiments of the invention incorporatematerials having relatively higher breakdown voltages, thus increasingthe overall breakdown voltage of the capacitor. As an example, materialsthat increase breakdown voltage can either be good electrical insulatorsor they can be very electrochemically inert (e.g., mercury). If thesematerials also have high dielectric constants (in which case they arereferred to herein as “high-k materials”), the materials may have theadditional beneficial effects of increasing capacitance and decreasingleakage current. Alternatively, separate layers or materials may be usedfor these purposes—i.e., one material to increase breakdown voltagealong with a separate high-k material. Charge storage devices usinghigh-breakdown-voltage materials in conjunction with porous structuresand organic electrolytes have much greater energy density than do chargestorage devices without such components.

A material is typically thought of as being a high-k material if itsdielectric constant is greater than the dielectric constant of SiO₂,i.e., greater than 3.9. Since some embodiments of the invention may useSiO₂ as a dielectric coating, SiO₂ (as well as any other materialshaving dielectric constants of 3.9) are explicitly included within thescope of “high-k materials” as defined herein. At the same time, itshould be noted that in other embodiments materials with significantlyhigher dielectric constants may also be used. To give several examples,the high-k material can be silicon nitride (SiN), silicon oxynitride(SiO_(x)N_(y)), hafnium oxide (HfO_(x)), zirconium oxide (ZrO_(x)),tantalum oxide (TaO_(x)), titanium oxide (TiO_(x)), or BaSrTiO₃ formedusing ALD, CVD, thermal growth, or wet chemistry, all of which havedielectric constants roughly on the order of 20-50. More exoticmaterials, with still higher dielectric constants (the values of whichare indicated in brackets next to each material below), may also beused. These include, for example, (LaSr)₂NiO₄ [10⁵], CaTiO₃ [10,286],and related materials such as CaCu₃Ti₄O₁₂ [10,286] and Bi₃Cu₃Ti₄O₁₂[1,871]. In some embodiments it may be desirable to select a high-kmaterial having a dielectric constant greater than that of theelectrolyte (often around 20 or so).

As suggested by the foregoing discussion of high-k materials, in someembodiments of the invention, charge storage device 100 furthercomprises a material having a dielectric constant of at least 3.9. Asillustrated in FIG. 5, which is a cross-sectional view of one ofchannels 111 of charge storage device 100 according to an embodiment ofthe invention, charge storage device 100 comprises a high-k material 515between electrolyte 150 and porous structure 110. (The EDL is not shownin FIG. 5 in order to avoid unnecessarily complicating the drawing.)

As mentioned above, embodiments of the invention increase thecapacitance of a charge storage device by increasing its surface areaand/or by decreasing the distance separating the conductive structures,and the preceding paragraphs have disclosed various techniques forachieving those results according to embodiments of the invention.According to additional embodiments, a capacitor's surface area may bestill further increased by the presence of nanostructures within atleast some of the channels of a charge storage device. (As used herein,the term “nanostructures” refers to structures having at least onedimension on the order of a nanometer up to a few tens of nanometers.Such nanostructures may be of regular or irregular shape.“Nanoparticles” are roughly spherical nanostructures. “Nanowires” aresolid, roughly cylindrical nanostructures. “Nanotubes” arenanostructures that also tend to be roughly cylindrical but differ fromnanowires in that they form hollow tubes. Carbon appears to be unique inits ability to form nanotubes; nanostructures made of other materialsform nanowires.)

In accordance with the foregoing discussion, and as shown in FIG. 5,channel 111 contains nanostructures 535. As an example, these can benanoparticles (perhaps in an isopropyl alcohol solution) or nanowires ofany suitable material (e.g., silicon) or combination of materials (e.g.,silicon germanium—with either a silicon core or a germanium core),carbon nanotubes, silicon-coated carbon nanotubes, or the like. Likechannels 111 and 121, as well as other portions of the porous structure,some (or all) of nanostructures 535 can in at least one embodiment becoated, or partially coated, with an electrically conductive coating540. As before, this coating should be a good electrical conductor(e.g., an appropriate metal, a silicide, or the like). At least some ofthe nanostructures may contain a dopant in order to further increasetheir electrical conductivity. Additionally, in some embodiments atleast some of nanostructures 535 are coated with a material 545 thatprevents an electrochemical reaction between nanostructures 535 andelectrolyte 150. Material 545 increases the breakdown voltage of thecharge storage device. As one example, material 545 may take the form ofa monolayer of mercury or of another liquid metal like gallium or agallium-indium-tin alloy on a surface of nanostructures 535 (or perhapsover electrically conductive coating 540, where such a coating ispresent).

Turning next to FIG. 6, a charge storage device 600 according to anotherembodiment of the invention will be discussed. As illustrated in FIG. 6,charge storage device 600 comprises a plurality of nanostructures 610 ona substrate 605, and further comprises an electrolyte 650 in physicalcontact with at least some of nanostructures 610. (In the illustratedembodiment, nanostructures 610 are discrete nanostructures, i.e., theyare, unlike the channels of a porous structure, for example, stand-alonestructures that are not contained within another structure.) As anexample, electrolyte 650 can be similar to electrolyte 150 that wasfirst shown in FIG. 1. The presence of electrolyte 650 creates an EDL;i.e., charge storage device 600 is an EDLC. As an example, a firstsubset of plurality of nanostructures 610 forms a first electrode ofcharge storage device 600 and a second subset of plurality ofnanostructures 610 forms a second electrode of charge storage device600.

A charge storage device made up simply of the nanostructures and theelectrolyte may represent a valuable, high-capacitance ultracapacitoraccording to an embodiment of the invention. As discussed above,however, it may often be desirable to increase the breakdown voltageand/or increase the capacitance and decrease the leakage current of thecharge storage device and thus, in certain embodiments, a high-kmaterial 615 (recall from above that this is defined herein as amaterial having a dielectric constant of at least 3.9) may be placedbetween electrolyte 650 and nanostructures 610. In the illustratedembodiment high-k material 615 takes the form of a coating at leastpartially covering the nanostructures. In some embodiments at least someof nanostructures 610 may additionally be coated with a material 645that prevents an electrochemical reaction between nanostructures 610 andelectrolyte 650. As an example, material 645 can be similar to material545 that is shown in FIG. 5 and can in one embodiment, therefore, takethe form of a monolayer of mercury (or one of the other substancesmentioned) on a surface of the nanostructures.

In certain embodiments nanostructures 610 are nanowires formed from asuitable material (e.g., silicon, silicon-germanium (SiGe), III-Vcompounds (such as gallium arsenide (GaAs) or the like), among manyothers). In other embodiments nanostructures 610 comprise carbonnanotubes.

FIG. 7 is a flowchart illustrating a method 700 of making anelectrically conductive structure for a charge storage device accordingto an embodiment of the invention.

A step 710 of method 700 is to provide a solution comprising a pluralityof nanostructures in a solvent. In one embodiment the solvent is aphotoresist material, especially a thick photoresist material (e.g., onthe order of 500 μm thick). In other embodiments the solution cancomprise a solvent other than photoresist. In a particular embodimentthe solution comprises conductive nanoparticles in isopropyl alcohol.Using photoresist as the solvent may be advantageous because it isalready so commonly used in microelectronics technology. Usingphotoresist may also simplify the patterning of the electricallyconductive structures formed according to method 700, if such patterningis desired. Another possible advantage of using photoresist as thesolvent arises when the nanostructures in the solvent are carbonnanotubes. In that case, the organic nature of the (carbon-based)photoresist leads to a high degree of compatibility with the organicnanotubes. The resulting carbon-carbon contacts in such a solution yielda high electrical conductivity.

A step 720 of method 700 is to apply the solution to a substrate. As anexample, the substrate could be made of silicon (perhaps heavily dopedsilicon), silicon or another material having a conductive film (e.g.,aluminum) deposited thereon, sheet glass coated with a thin film ofmetal, or, more generally, any suitable conductive material that issufficiently rigid to act as a support. In one embodiment step 720comprises electrospinning a photoresist material onto the substrate.Electrospinning involves applying an electric charge so that the fibersor other nanostructures can be directed into a desired arrangement. Inone embodiment electrospinning the photoresist material creates aplurality of fibers at least some of which have a length of at least 500μm. Regular spinning (without an electric charge) is an alternative toelectrospinning Either spinning procedure permits a nice uniformapplication of the solution on the substrate. Alternatively, thesolution could simply be poured onto the substrate without spinning,although thickness would likely be harder to control with thistechnique—the needed amount of solution would have to be very carefullymeasured out and accounted for to make sure none spilled off the edge ofthe wafer or substrate.

A step 730 of method 700 is to anneal the solution and the substrate inorder to form the electrically conductive structure. Annealing drivesout the solvents and leaves behind a structure that can be relativelythick. In one embodiment the anneal can comprise a pyrolysis reaction.If desired, some of the solvents can be driven out in a preliminary,lower-temperature event—perhaps by baking the substrate in an oven—thatat least partially hardens the solution.

A step 740 of method 700 is to form a dielectric material on at leastsome of the nanostructures in order to improve the breakdown voltage.This could be accomplished, for example, by depositing aluminum oranother suitable material on the nanostructures and then oxidizing thealuminum or other material.

The performance of method 700 results in an electrically conductivestructure that may have a relatively substantial thickness. In order toachieve robust capacitance, a capacitive structure should have agenerous thickness, a fact that existing capacitive structures based onnanostructures tend to ignore. Method 700, in at least some embodiments,thus makes use of a thick, organic photoresist (e.g., SR8) that can bespun onto a substrate at thicknesses of 500 μm or more. After pyrolysis,which drives out the solvent, one is left with nanostructures formedinto a high-surface-area structure with a thickness on the order of theoriginal thickness of the photoresist. An illustrative structure of thetype described is shown in FIG. 8, where nanostructures 820 on asubstrate 810 are visible. In this regard, it should be noted that FIG.8, like some of the preceding figures, is an idealized representation ofa structure that in reality would be much less orderly and much morelike a jumbled haystack or a honeycomb or the like.

An alternative method for making a thick electrically conductivestructure is by using nanoimprint lithography. This method involvescreating a stencil that is then physically pushed up against aphotoresist or the like, which forms the photoresist material intovalleys and plateaus. This method likely will not result in thicknessesfor the electrically conductive structure as great as those achievablewith method 700, but will likely result, nevertheless, in fairly robustthicknesses on the order of perhaps 50-100 μm.

FIG. 9 is a flowchart illustrating a method 900 of making a chargestorage device according to an embodiment of the invention.

A step 910 of method 900 is to provide an electrically conductivestructure having a first section and a second section. In one embodimentstep 910 comprises providing a solution comprising a plurality ofnanostructures in a solvent, applying the solution to a substrate, andannealing the solution and the substrate in order to form theelectrically conductive structure.

A step 920 of method 900 is to place a membrane or other separatorbetween the first section and the second section, wherein the separatorallows a transfer of ionic charge. In one embodiment step 920 or anotherstep further comprises etching a trench between the first section andthe second section and placing the separator in the trench.

A step 930 of method 900 is to place an electrolyte in physical contactwith the electrically conductive structure.

FIG. 10 is a block diagram representing a mobile electronic device 1000according to an embodiment of the invention. As illustrated in FIG. 10,mobile electronic device 1000 comprises a substrate 1010 on which amicroprocessor 1020 and a charge storage device 1030 associated withmicroprocessor 1020 are disposed. Charge storage device 1030 can eitherbe located on substrate 1010 away from microprocessor 1020, asillustrated in solid lines, or it can be located on microprocessor 1020itself, as illustrated in dashed lines. In one embodiment charge storagedevice 1030 comprises first and second electrically conductivestructures separated from each other by an electrical insulator, whereat least one of the first and second electrically conductive structurescomprises a porous structure containing multiple channels. As anexample, this embodiment can be similar to one or more of theembodiments shown in FIGS. 1-5 and described in the accompanying text.In another embodiment charge storage device 1030 comprises a pluralityof nanostructures (e.g., discrete nanostructures) and an electrolyte inphysical contact with at least some of the nanostructures. As anexample, this embodiment can be similar to one or more of theembodiments shown in FIG. 6 and described in the accompanying text.

In at least some embodiments charge storage device 1030 is one of aplurality of charge storage devices (all of which are represented inFIG. 10 by block 1030) contained within mobile electronic device 1000.In one or more of those embodiments mobile electronic device 1000further comprises a switching network 1040 associated with the chargestorage devices. When a capacitor is being discharged it doesn'tmaintain a constant voltage but instead decays in an exponential manner(unlike a battery where the voltage stays relatively constant duringdischarge). Switching network 1040 comprises circuitry or some othermechanism that switches in and out various capacitors such that arelatively constant voltage is maintained. For example, the chargestorage devices could initially be connected to each other in paralleland then, after a certain amount of voltage decay, a subset of thecharge storage devices could be changed by the switching network so asto be connected in series such that their individual voltagecontributions can boost the declining overall voltage. In one embodimentswitching network 1040 could be implemented using existing silicondevice technology as used in the art (transistors, silicon controlledrectifiers (SCRs), etc.), while in other embodiments it could beimplemented using micro-electromechanical systems (MEMS) relays orswitches (which, it may be noted, tend to have very low resistance).

In some embodiments mobile electronic device 1000 further comprises asensor network 1050 associated with charge storage devices 1030. In atleast some embodiments each one of the plurality of charge storagedevices will have its own sensor that indicates certain behavioralparameters of the charge storage device. For example, the sensors mayindicate existing voltage levels as well as the ongoing dischargeresponse, both of which are parameters that may be used by the switchingnetwork—especially in cases where the dielectric material (or otherelectrical insulator) being used is not linear but rather has adielectric constant that varies with the voltage. In those cases, it maybe advantageous to include along with the sensor network a finite statemachine such as a voltage control unit 1060 that knows what the behaviorof the dielectric is and responds accordingly. A voltage control unitthat knows how the dielectric behaves can compensate for anynon-linearity. A temperature sensor 1070 associated with charge storagedevices 1030 may also be included in order to sense temperature (orother safety-related parameters). In certain embodiments of theinvention, mobile electronic device 1000 further comprises one or moreof: a display 1081, antenna/RF elements 1082, a network interface 1083,a data entry device 1084 (e.g., a keypad or a touchscreen), a microphone1085, a camera 1086, a video projector 1087, a global positioning system(GPS) receiver 1088, and the like.

FIG. 11 is a block diagram representing a microelectronic device 1100according to an embodiment of the invention. As illustrated in FIG. 11,microelectronic device 1100 comprises a substrate 1110, a microprocessor1120 over substrate 1110, and a charge storage device 1130 associatedwith microprocessor 1120. Charge storage device 1130 can either belocated on substrate 1110 away from microprocessor 1120 (e.g., adie-side capacitor), as illustrated in solid lines, or it can be locatedon microprocessor 1120 itself (e.g., in a build-up layer above themicroprocessor), as illustrated in dashed lines. In one embodimentcharge storage device 1130 comprises first and second electricallyconductive structures separated from each other by an electricalinsulator, where at least one of the first and second electricallyconductive structures comprises a porous structure containing multiplechannels. As an example, this embodiment can be similar to one or moreof the embodiments shown in FIGS. 1-5 and described in the accompanyingtext. In another embodiment charge storage device 1130 comprises aplurality of nanostructures (e.g., discrete nanostructures) and anelectrolyte in physical contact with at least some of thenanostructures. As an example, this embodiment can be similar to one ormore of the embodiments shown in FIG. 6 and described in theaccompanying text.

The charge storage devices disclosed herein may in some embodiments beused as a decoupling capacitor within microelectronic device 1100—onethat is smaller and that, for the reasons described elsewhere herein,offers much higher capacitance and much lower impedance than existingdecoupling capacitors. As already mentioned, charge storage device 1130can be part of a support integrated circuit (IC) or chip or it can belocated on the microprocessor die itself. As an example, one might,according to embodiments of the invention, be able to form regions ofporous silicon (or the like, as described above) on a microprocessor dieand then create a high-surface-area embedded decoupling capacitor righton the substrate of the microprocessor die. Because of the porosity ofthe silicon, the embedded capacitor would have very high surface area.Other possible uses for the disclosed charge storage devices include useas a memory storage element (where problems with the z-direction size ofembedded DRAM approaches may be solved by greatly increasing the faradsper unit area) or as a component of voltage converters in voltage boostcircuitry, perhaps for use with circuit blocks, individualmicroprocessor cores, or the like.

As an example, higher capacitance values could in this context beadvantageous because parts of the circuit could then run nominally at acertain (relatively low) voltage but then in places where higher voltageis needed in order to increase speed (e.g., cache memory, input/output(I/O) applications) the voltage could be boosted to a higher value. Anoperational scheme of this sort would likely be preferred over one inwhich the higher voltage is used everywhere; i.e., in cases where only asmall amount of circuitry requires a higher voltage it likely would bepreferable to boost voltage from a lower baseline voltage for that smallportion of the circuit rather than drop voltage from a higher baselinevalue for the majority of the circuitry. Future microprocessorgenerations may also make use of voltage converters of the typedescribed here. Having more capacitance available to be deployed arounda package or around a microprocessor die may help solve the existingissue of intolerably high inductance between transistors that transfervoltage around a circuit.

Although the invention has been described with reference to specificembodiments, it will be understood by those skilled in the art thatvarious changes may be made without departing from the spirit or scopeof the invention. Accordingly, the disclosure of embodiments of theinvention is intended to be illustrative of the scope of the inventionand is not intended to be limiting. It is intended that the scope of theinvention shall be limited only to the extent required by the appendedclaims. For example, to one of ordinary skill in the art, it will bereadily apparent that the charge storage devices and the relatedstructures and methods discussed herein may be implemented in a varietyof embodiments, and that the foregoing discussion of certain of theseembodiments does not necessarily represent a complete description of allpossible embodiments.

Additionally, benefits, other advantages, and solutions to problems havebeen described with regard to specific embodiments. The benefits,advantages, solutions to problems, and any element or elements that maycause any benefit, advantage, or solution to occur or become morepronounced, however, are not to be construed as critical, required, oressential features or elements of any or all of the claims.

Moreover, embodiments and limitations disclosed herein are not dedicatedto the public under the doctrine of dedication if the embodiments and/orlimitations: (1) are not expressly claimed in the claims; and (2) are orare potentially equivalents of express elements and/or limitations inthe claims under the doctrine of equivalents.

What is claimed is:
 1. A charge storage device comprising: a firstelectrically conductive structure and a second electrically conductivestructure separated from each other by an electrical insulator, wherein:at least one of the first electrically conductive structure and thesecond electrically conductive structure comprises a porous structurecontaining multiple channels; the charge storage structure furthercomprises nanostructures located within at least some of the channels ofthe porous structure; and each one of the channels has an opening to asurface of the porous structure.
 2. The charge storage device of claim 1wherein: a smallest dimension of each one of the channels is no lessthan 2 nanometers.
 3. The charge storage device of claim 1 wherein: thesmallest dimension of each one of the channels is no greater than 1micrometer.
 4. The charge storage device of claim 1 wherein: the porousstructure is made of a material selected from the group comprisingsilicon, germanium, silicon-carbide, silicon-germanium, aluminum,tungsten, and copper.
 5. The charge storage device of claim 1 furthercomprising: an electrically conductive coating on at least a portion ofthe porous structure and in at least some of the channels.
 6. The chargestorage device of claim 1 wherein: the porous structure contains adopant.
 7. The charge storage device of claim 1 wherein: the electricalinsulator comprises a dielectric material.
 8. The charge storage deviceof claim 1 further comprising: an electrolyte in physical contact withthe porous structure, wherein: the electrical insulator is a doublelayer created by the presence of the electrolyte.
 9. The charge storagedevice of claim 8 wherein: the electrolyte is an organic electrolyte.10. The charge storage device of claim 8 further comprising: a materialhaving a dielectric constant of at least 3.9 between the electrolyte andthe porous structure.
 11. The charge storage device of claim 1 furthercomprising: an electrically conductive coating on at least a portion ofat least some of the nanostructures.
 12. The charge storage device ofclaim 1 further comprising: an electrolyte in physical contact with theporous structure.
 13. The charge storage device of claim 12 wherein: atleast some of the nanostructures are coated with a material thatprevents an electrochemical reaction between the nanostructures and theelectrolyte.
 14. The charge storage device of claim 12 wherein: thematerial is one of mercury, gallium, and gallium-indium-tin; and thematerial forms a monolayer on a surface of the nanostructures.
 15. Thecharge storage device of claim 1 wherein: at least some of thenanostructures contain a dopant.
 16. A mobile electronic devicecomprising: a microprocessor; and a charge storage device associatedwith the microprocessor, wherein: the charge storage device comprises afirst electrically conductive structure and a second electricallyconductive structure separated from each other by an electricalinsulator; and at least one of the first electrically conductivestructure and the second electrically conductive structure comprises aporous structure containing multiple channels, at least some of whichcontain nanostructures located therein.
 17. The mobile electronic deviceof claim 16 wherein: the charge storage device is one of a plurality ofcharge storage devices; and the mobile electronic device furthercomprises a switching network associated with the charge storagedevices.
 18. The mobile electronic device of claim 17 furthercomprising: a sensor network associated with the charge storage devices.19. The mobile electronic device of claim 18 further comprising: avoltage control unit associated with the sensor network and with thecharge storage devices.
 20. The mobile electronic device of claim 19further comprising: a temperature sensor associated with the chargestorage devices.
 21. A charge storage device comprising: a firstelectrically conductive structure and a second electrically conductivestructure separated from each other by an electrical insulator, wherein:at least one of the first electrically conductive structure and thesecond electrically conductive structure comprises a porous structurecontaining multiple channels; the charge storage device furthercomprises an electrolyte in physical contact with the porous structure;the electrical insulator is a double layer created by the presence ofthe electrolyte; the charge storage device further comprises a materialhaving a dielectric constant of at least 3.9 between the electrolyte andthe porous structure; and each one of the channels has an opening to asurface of the porous structure.